Two common causes.
One, as per Khandro's post, is that any bath/shower/basin traps can lose their water seal through evaporation. Just need topping up.
The other one is something that does result in a really strong sewage smell. You may possibly have an "air admittance valve" in the corner of the room, usually to the side of the WC.
This allows air to get into the drains to avoid syphonage.
These can get stuck open, resulting in drain odours.
Do you have a, approx. 6"x6", boxed-in soil pipe in the corner?
If so, does it go right up to the ceiling, or stop halfway up?